It is known that more lithium ions per unit volume can be intercalated into silicon materials, such as silicon (Si) and silicon oxides represented by SiOx, than into carbon materials, such as graphite. In particular, the volume change due to the intercalation of lithium ions is smaller in SiOx than in Si, and application of SiOx to a negative electrode of lithium-ion batteries has been investigated. For example, Patent Literature 1 discloses a non-aqueous electrolyte secondary battery that contains a mixture of SiOx and graphite as a negative-electrode active material.
However, non-aqueous electrolyte secondary batteries that contain SiOx as a negative-electrode active material have lower initial charge/discharge efficiency than non-aqueous electrolyte secondary batteries that contain graphite as a negative-electrode active material. This is mainly because SiOx is converted into Li4SiO4 (an irreversible reactant) in an irreversible reaction during charging and discharging. Thus, in order to suppress such an irreversible reaction and improve initial charge/discharge efficiency, a negative-electrode active material represented by SiLixOy (0<x<1.0, 0<y<1.5) is proposed (see Patent Literature 2). Patent Literature 3 discloses a negative-electrode active material containing a lithium silicate phase composed mainly of Li4SiO4 in silicon oxide.